Conversion of metal oxidation states by phytoreduction

ABSTRACT

The present invention provides a method for remediating soil contaminated with Cr(VI) by reducing the Cr(VI) to Cr(III) in the soil. The method involves contacting the contaminated soil with a plant that accomplishes the reduction. Removal of the plant from the environment is not required; in fact, preferred embodiments of the invention involve plowing the plant back into the soil environment and replanting the soil.

The invention was made with U.S. government support under Grant No. R818619-01-0 awarded by the U.S. Environmental Protection Agency toRutgers, the State University of New Jersey. The government has certainrights in the invention.

The present invention is a continuation-in-part of co-pendingapplication Ser. No. 08/252,234 entitled "Phytoremediation of Metals",filed Jun. 1, 1994, which is a continuation-in-part of U.S. Ser. No.08/073,258, filed Jun. 4, 1993, and issued as U.S. Pat. No. 5,364,451 onNov. 15, 1994.

BACKGROUND OF THE INVENTION

Deposition of metal-rich mine tailings, metal smelting, leather tanning,electroplating, emissions from gas exhausts, energy and fuel production,downwash from powerlines, intensive agriculture and sludge dumping arethe most important human activities which contaminate soil systems withlarge amounts of toxic metals. The list of sites contaminated with toxicmetals grows larger every year, presenting a serious health problem anda formidable danger to the environment. In spite of the growing numberof metal-contaminated soil sites, the costly process of removing andburying metal-contaminated soils, or isolating the contaminated sites,remain the most commonly used methods for reclaiming metal-contaminatedsoils.

Moreover, many heavy metals exist in situ in their anionic form and someof these metals may pose unique remediation problems. For instance,chromium exists in soil in two different oxidation states; cationic Cr⁺³Cr(III)! and its oxidized, hexavalent anionic form Cr⁺⁶ Cr(VI)!. Bothspecies of chromium have very different properties. Reduced cationicCr(III) is insoluble in soils, and is therefore unable to move into thefood chain. Because it is not readily available to be included in thefood chain and thus its toxicity is inherently low, reduced Cr(III)posses only limited health risks.

On the other hand, anionic chromium (VI) is readily leachable and ismobile in soil and may be taken up by plants or released into thegroundwater. It can move much more easily into the food chain than itsreduced species and is capable of producing toxicity in humans and otheranimals. A dramatic reduction in the toxicity of Cr(VI) in soil could beachieved by finding a way to efficiently convert Cr(VI) to Cr(III).

SUMMARY OF THE INVENTION

The present invention pertains to a method of removing cationic andanionic forms of metal from a metal-containing soil environment usingterrestrial plants, most preferably of the family Brassicaceae(i.e.,"phytoremediation"). In order to accomplish this, at least onemember of the family Brassicaceae is contacted with the metal-containingsoil environment and the metal-containing soil environment ismanipulated in a manner sufficient to increase availability of metalwithin the metal-containing soil environment to the member of the plantfamily Brassicaceae. The plant member is maintained in themetal-containing soil environment under conditions of increased metalavailability for a time and under conditions sufficient for the memberto accumulate an amount of metal from the metal-containing soilenvironment (i.e., "phytoaccumulation").

In one embodiment, the step of manipulating the soil includes tillingthe soil to a depth greater than about 15 cm so that metal-containingsoil is brought into contact with the root zone of the Brassicaceae.Preferably, the soil is tilled to a depth of about 50 cm. In anotherembodiment, the soil is excavated to depths greater than about 20 cm andplaced in elevated seed beds or hills. The Brassicaceae member isplanted directly on the excavated soil in the hills and, when fullygrown, the roots are harvested.

Another embodiment entails adding a chelating agent to the soil in anamount sufficient to form a soluble or insoluble complex with at leastone divalent metal in the soil. Further, an electric field may beapplied to the soil to increase metal mobility. The methods also includedecreasing pH of the metal-containing soil to at least pH 5.5 or less byadding an effective amount of an organic or inorganic acid such as, forexample, nitric acid, hydrochloric acid, sulfuric acid, acetic acid, andcitric acid. Further manipulative methods include addition of a compoundto the soil that will be metabolized by the roots and/or associatedbacteria (collectively called the "rhizosphere") with concomitantproduction of protons, leading to a decrease in soil pH.

Further methods of the invention include foliar fertilization of theBrassicaceae with a phosphate fertilizer, preferably at a rate of about10-18 kg/hectare. Other methods include harvesting the Brassicaceaemember from the metal-containing soil environment before seeds of theBrassicaceae mature and harvesting roots of the Brassicaceae member.

Preferred plants used in the present methods are crop members andcrop-related members of the family Brassicaceae selected for an abilityto accumulate at least 10 times more metal in shoots on a dry weightbasis that the amount of metal present in the metal-containing soil.Selected plant members are also able to accumulate at least 20 timesmore metal in roots on a dry weight basis that the amount of metalpresent in the metal-containing soil. Preferred crop members areselected from the group consisting of Brassica juncea and Brassicacarinata. Preferred crop-related members are selected from the groupconsisting of Raphanus sativus (L.) (radish), Sinapis alba (L.) (whitemustard). S. arvensis (L.), S. flexuosa Poiret and S. pubescens (L.).

The present invention is also directed to a method of converting asoluble form of ionic chromium to an insoluble form of ionic chromium ina soil environment containing the soluble form. The method includesinitiating a first planting of a plant capable of converting a solubleform of ionic chromium to an insoluble form of ionic chromium andmaintaining the plant in the chromium-containing soil environment for atime and under conditions sufficient for the plant to convert thesoluble ionic form of chromium to the insoluble ionic form of chromiumin the plant by reducing the soluble ionic form of chromium. The plantis then manipulated to increase reduction of soluble chromium withoutremoving said plant from the soil.

The present invention therefor pertains to a method of reducing anamount of soluble hexavalent chromium, (Cr₂ O₇)⁻² : hereinafter Cr(VI)!in chromium (VI)-containing environments to insoluble trivalent chromiumhereinafter Cr(III)! using plants, most preferably plants of the familyBrassicaceae (i.e., "phytoreduction"). In one preferred embodiment, afirst planting in a Cr(VI)-containing soil environment is initiated andthe plant is maintained in the Cr(VI)-containing soil environment for atime and under conditions sufficient for it to reduce Cr(VI) in theplant to chromium (III). In effect, methods contemplated by the presentinvention use plants in a chemical engineering sense as reducing agentsto render the soil chromium essentially unavailable to be leached orotherwise transported through the soil. In preferred methods, the stepof maintaining the plant comprises growing the member of the familyBrassicaceae for about 2-3 months in the Cr(VI)-containing soil and thestep of manipulating comprises plowing the member into the soil andreplanting the soil with a second planting of a member of the familyBrassicaceae under conditions sufficient for the newly plantedBrassicaceae member to accumulate Cr(VI) from the soil into the plantand reduce Cr(VI) in the plant to Cr(III).

Metals that may be phytoaccumulated by the present methods includeanionic and cationic forms of lead, chromium, cadmium, mercury, cobalt,barium, nickel, molybdenum, copper, zinc, antimony, beryllium, gold,manganese, silver, thallium, tin, rubidium, strontium, yttrium,technetium, ruthenium, palladium, indium, cesium, uranium, plutonium,and cerium. Metals that may be phytoreduced include chromium, selenium,arsenic, and vanadium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a bar graph illustrating accumulation of lead by Brassicajuncea cultivars in shoots (FIG. 1A) and roots (FIG. 1B).

FIG. 2 is a bar graph illustrating stabilization of lead by B. juncea.

FIG. 3 is a bar graph illustrating accumulation of various metals by B.juncea and A. paniculata. Levels of metal applied (microgram metal/gramsoil) are shown in parenthesis next to the metal symbol."Phytoextraction coefficient" is the ratio of metal concentration inroot tissue (dry weight basis) to metal concentration in soil (dryweight basis).

FIG. 4 is a graph illustrating the removal of Cr(VI) from solution byroots of B. juncea.

FIG. 5 is a graph illustrating the XAS of B. juncea roots exposed tosolutions of Cr(VI). Samples of roots and leaves were analyzed asdescribed above and the XAS compared with standards including chromiumas: (i) Cr(VI) in the form of K₂ CrO₄ ; (ii) Cr(III) in the form ofCr(NO₃)₃ and Cr₂ O₃ ; and (iii) elemental chromium.

DETAILED DESCRIPTION OF THE INVENTION

I. Phytoaccumulation of Cationic and Anionic Metal Species

One aspect of the present invention is a method for removing metals frommetal-contaminated soil using metal-accumulating plants of the familyBrassicaceae. This is accomplished by manipulating the soil environmentin a way that increases availability of metal to the plant. Themanipulations described herein (See Example 1) are generally designed tomaximize metal uptake by plants, something that would be consideredantithetical to current practices and counterproductive by conventionalagronomists. Specifically, manipulating the soil environment accordingto the invention and then growing one more members of these Brassicaceaeunder conditions sufficient for the plants to accumulate metal in theirbiomass is generally contrary to current agricultural practices forgrowing crop and crop-related Brassicas. See texts describingconventional practices, such as for example, Chopra, V. L. and Prakash,S., (eds.) Oilseed Brassicas in Indian Agriculture, Vikas PublishingHouse Ltd., New Delhi, (1991); Downey, R. K. and Robbelen, G., "Brassicaspecies", pp. 339-362 in Robbellen et al. (eds.), Oil Crops of theWorld, McGraw-Hill, New York, 1989, incorporated herein by reference.

In brief, preferred methods of the invention involve soil and/or plantmanipulations that impact on crop establishment and tillage, soilacidity and soil fertility. The Brassicaceae manipulated in this mannermay include genetically altered plants, as described herein. In thiscontext, metal "accumulating" plants refers to the ability of the plantsdescribed herein to perform one, or more, of the following activities:(i) transporting anionic and cationic metal species from soil particlesand/or soil liquid into roots; (ii) physical and/or chemical sorption ofcationic and anionic metal species to the root biomass; (iii) preventionor inhibition of leaching of the cationic and anionic metal species fromthe soil environment. The term "increased availability of metal" refersto the ability of the present methods to render metals in soils moreamenable to plant root uptake than they would be absent the presentmethods.

The term "metal" preferably refers to cationic and anionic metal ionsthat are found in the metal containing environment. It will beappreciated that this term will also include elemental metal that is notin an ionic form. The metals that can be accumulated according to themethod of the present invention include stable metals and radioactivemetals that exist as cations in soils such as lead, mercury, cadmium,cobalt, barium, nickel, molybdenum, copper, zinc, antimony, beryllium,gold, manganese, silver, thallium, tin, rubidium, strontium, yttrium,technetium, ruthenium, palladium, indium, cesium, uranium, plutonium,and cerium. The term "metal" also refers to anionic metal species suchas those of chromium, arsenic, selenium, and vanadium.

The term "metal" is also intended to include more than one metal sinceplants may concentrate several different metals (See Example 6),implying that the mechanism of metal uptake is not always metalspecific.

The term "metal" also includes mixtures of metals and common organicpollutants such as, for example, lead or chromium in combination withnitrophenol, benzene, alkyl benzyl sulfonates (detergents),polychlorinated biphenyls (PCB's) and/or halogenated hydrocarbons (e.e.,trichloroethylene).

The preferred plants used in the present method are members of the plantfamily Brassicaceae. The most preferred members of this family belong tothe tribe Brassiceae. Members of this tribe include mustards of thegenus Brassica and related species, described in more detail below.

A key aspect of the present invention is that the preferred methodrelies upon use of crop and/or crop-related members of theabove-identified family and tribe. The term "crop member" refersspecifically to species of the genus Brassica which are commerciallygrown as sources for primarily two different types of products: (i)vegetables, forage, fodder and condiments; and (ii) oilseeds. Examplesof "vegetative" crop members of the family Brassicaceae include, but arenot limited to, digenomic tetraploids such as Brassica juncea (L.)Czern. (mustard), B. carinata Braun (ethopian mustard), and monogenomicdiploids such as B. oleracea (L.) (cole crops), B. nigra (L.) Koch(black mustard) and B. campestris (L.) (turnip rape). Examples of"oil-seed" crop members of the family Brassicaceae include, but are notlimited to, B. napus (L.) (rapeseed), B. campestris (L.), B. juncea (L.)Czern. and B. tournifortii.

"Crop-related" members are those plants which have potential value as acrop and as donors of agronomically useful genes to crop members. Thus,crop-related members are able to exchange genetic material with cropmembers, thus permitting breeders and biotechnologists to performinterspecific (i.e., from one species to another) and intergeneric(i.e., from one genus to another) gene transfer. Those having ordinaryskill in the art will understand that methods of exchanging geneticmaterial between plants and testing effects of interspecific andintergeneric gene transfer are well characterized. See, for exampleGoodman et al., Science, 236: 48-54, 1987, incorporated herein byreference.

"Crop-related" members include members of species belonging, but notlimited to, genera of Sinapsis, Thlaspi, Alyssum, and Eruca Raphanus."Crop-related" members not presently identified, or suspected ofremoving metal, can be identified using the screening methods describedherein. Unless indicated otherwise, "crop and/or crop-related" memberswill be referred to collectively as "members".

The plant members used in the present methods include mutagenized and/orgenetically engineered plants (i.e, interspecific and/or intergenerichybrids). For example, ethylmethylsulfonate (EMS) is a potent mutagenwhich increases genetic variability by increasing the frequency ofgenomic mutations. See, for example, Redei, G. P. "Genetic Manipulationsof Higher Plants", L. Ledoux (ed), Plenum Press, New York, 1975.Ethylmethylsulfonate has been used in selection programs to produceheritable changes in plant biochemistry and physiology, particularly inArabidopsis thaliana, a member of the Brassicaceae.

In sum, the members used in the present invention are plants that: (a)can be grown to high biomass; (b) are adaptable for growth in variousagroclimatic conditions; (c) are adaptable to modified, non-conventionalagricultural practices, described herein, for monoculture; (d) areamenable to genetic manipulation by mutagenesis and/or gene transfer;(e) can produce several crops per year; and (f) are related to knownwild plants which do accumulate metals.

Preferred plant members used in the present invention should becontrasted to "wild" or non-crop and/or non-crop-related members; i.e.,those species that are endemic to metal-containing soils in scatteredareas of the world. These wild members are not amenable to large scaleagricultural practices and they normally have very low rates ofgermination and biomass accumulation in the laboratory and in the field.

Examples of non-crop-related members of the family Brassicaceae aremembers of the genus Alyssum found on serpentine soils in southernEurope and Thlaspi from calamine soils throughout Europe. In particular,non-crop-related members of this family include T. caerulescensWhitesike Mine, A. tenium, A. lesbiacum, A. murale and T. ochroleucum(see also Baker, et al., Biorecovery 1:81-126 (1989); Reeves and Brooks,Environ. Poll., 31: 277 (1983); Baker et al., Taxon, 34: 89 (1985)).

In one embodiment of the method, a screening system (described inExample 3) is used to identify terrestrial plant species with thehighest metal accumulating potential (i.e. metal content of dried plantresidue/metal content of growth medium). The seeds of theseself-pollinating lines are then subjected to EMS mutagenesis using, forexample, the methods of Estell et al, "The mutants of Arabidopsis", p.89 in Trends in Genetics, Elsevier Science Publishers, B. V., Amsterdam,1986. (See Example 9). Briefly, mutagenesis is accomplished by soakingdry seeds in EMS solution at room temperature. The EMS inducesheterozygous mutations in those cells which will produce thereproductive structures. The M1 generation of plants is allowed toself-fertilize and at least 50,000 seedlings of the M2 progeny arescreened for metal tolerance in artificial aqueous solutions containingvarious metal concentrations. The most tolerant M2 plants, those growingmost vigorously, are analyzed for accumulation of metals.

The terrestrial plants used in the methods of the present invention canbe genetically manipulated using well-established techniques for genetransfer. It is well-known that a variety of non-photosyntheticorganisms respond to metals by production of metallothioneins (MT's),low molecular weight proteins encoded by structural genes. See, forexample G. Maroni, "Animal Metallothioneins," pp. 215-232 in Heavy MetalTolerance in Plants: Evolutionary Aspects, (ed. A. J. Shaw), CRC Press,Inc., Florida, (1990). The present invention contemplates increasingroot uptake of metals by heterologous expression of MT's in transgenicplants.

In another embodiment of the method, a mammalian MT cDNA (e.g. monkey)can be obtained commercially or from an established source and arestriction enzyme fragment cloned into, for example, anAgrobacterium-based plant transformation/expression vector such aspJB90, a derivative of pGSFR780A. See, De Block et al, Physiol. Plant91: 694-701 (1989).

Seedling segments of terrestrial plants used in the present method arethen incubated in the presence of a suspension of bacterial cells (e.g.Agrobacterium tumefacieus) carrying the expression vector. After severaldays, the regenerating seedling segments are transferred to theappropriate selection medium and further incubated. This results intransformants containing the mammalian MT genome (see Example 7).

The transformants are analyzed for the presence of MT DNA by Southernand Northern hybridization using mammalian MT as the probe. Thetransformants are also analyzed for expression of MT protein byimmunoblot analysis with antisera against the mammalian MT. Seeestablished protocols of, for example, Sambrook et al. (1989) MolecularCloning: A Laboratory Manual, Cold Spring Harbor Laboratory Press, NewYork, incorporated herein by reference. Sexual and asexual (i.e.,somatic) hybridization is another way of introducing metal-accumulatingtraits into members of the Brassicaceae. Hybridization has been used totransfer agronomically important traits from related species to cropbrassicas. See, for example, Salisbury and Kadkol, Genetics (Life Sci.Adv. 8: 65-87 (1989).

The metal-containing environment into which these plants are introducedis not intended to limit the scope of the invention. That is, as long asthe environment can sustain growth of members of the familyBrassicaceae, the metal-containing environment includes a wide range ofsoil environments of varying degrees of water saturation, organic mattercontent, mineral content, and the like. It will be appreciated by thoseof ordinary sill in the art that the term "soil" can, therefore, includea wide variety of chemical and physical types.

The phytoaccumulating members suitable for the present methods willextract metal from the environment into the roots of the plant. Theplants may translocate the metals from the roots into the shoots (i.e.,the above ground portions of the plant). The rates of accumulation ofmetal can vary depending on a variety of factors, including the ratio ofsoluble and insoluble metal in the soil, the total metal concentration,soil type, pH, moisture content, organic matter content, soiltemperature, planting density, and fertilizer use.

Generally, metal phytoaccumulation by the preferred members of thefamily Brassicaceae can be as high as 1000-fold above levels present inthe soil. Preferred plant members accumulate several percent of metal asdry weight of shoot biomass and up to 30% metal by weight in dried rootbiomass. Particularly preferred are those plants selected for theirability to accumulate at least 10 times more metal in shoots on a dryweight basis than the metal present in the metal-containing soil and/orat least 20 times more metal in roots on a dry weight basis than themetal present in the metal-containing soil. Shoots are routinelyharvested for certain Brassica species, for example B. campestris, B.juncea and B. oleracea, but roots are not routinely harvested,especially for most oil-seed Brassicaceae.

The members of the family Brassicaceae of the present invention haveundergone screening and selection procedures to yield several lines offast growing metal phytoaccumulating plants that can effectively removeradioactive and non-radioactive metals from artifactual and naturalsoils. These plants concentrate metals in roots and transport the metalsto the above-ground shoots which can be easily harvested.

The screening procedures detailed in Example 3 can be applied to othermembers of the family Brassicaceae and other metal ions that are notdescribed here. To measure metal phytoaccumulation of any plant in ametal-containing soil, seeds of the particular plant(s) to be tested aregrown in a greenhouse, the appropriate metal is administered to theplant and soil, and the roots and shoots harvested for routinedetermination of biomass and metal content. Chemical analysis of metalcontent in soils and plants is well-characterized. See, for example,Blincoe et al., Comm. Soil. Plant Anal., 18: 687 (1987); Baker, D. E.and Suhr, S. H., "Atomic Absorption Spectrometry", pp. 13-27 in Methodsof Soil Analysis, part 2, Am. Soc. Agron., Madison, Wis., (1982). Metalin plant tissues is preferably assayed with plasma spectrometry,following ashing and acid extraction. Metal remaining in the solution ismeasured by, for example, atomic absorption or plasma spectrometry. See,Soltanpour et al., "Optical emission spectrometry", pp.29-65 in Methodsof Soil Analysis, part 2, Am. Soc. Agron., Madison, Wis., (1982).

II. Phytoreduction of Hexavalent Chromium

As shown in FIG. 4 anionic chromium (VI) (Cr₂ O₇)⁻² ! is removed from asolution containing roots of Brassicaceae and the hexavalent chromium isaccumulated, primarily by the roots (See Example 5).

In addition, Cr(VI) is also reduced to Cr(III) by Brassicaceae. X-rayabsorbance spectroscopic analysis of the oxidation state of chromium inB. juncea roots previously immersed for 3 days in a solution containing3 mg/l chromium Cr(VI) as performed at the Stanford Synchotron radiationLaboratory (SSRL) at Menlo Park, Calif. (See Example 8). X-rayabsorbance spectroscopy demonstrated that roots of this plant exposed tochromium (VI) contained only chromium (III), indicating that roots ofthis plant could effectively reduce chromium (VI) to Cr(III). See FIG.5.

Members of the plant family Brassicaceae therefore reduce Cr(VI) toCr(III) in the plant. Another aspect of the present invention is thus amethod for reducing Cr(VI) in Cr(VI)-contaminated soil using plants ofthe family Brassicaceae. This is significant insofar as Cr(VI) is highlymobile in the soil environment but in its reduced state of Cr(III) it ismuch less mobile. In soils, Cr(III) is insoluble above pH 4 (Bartlettand Kimble, J. Environ. Qual., 5: 379-386, 1976). Thus, in most soilsCr(III) is unavailable for leaching or plant uptake. However, Cr(VI) issoils is either soluble or loosely bound to the soil matrix (James andBartlett, J. Environ. Qual., 12: 177-181, 1983) and is much more readilyleached and taken up by plants.

The manipulations described herein (See Example 2) are generallydesigned to maximize Cr(VI) reduction by plants so that the soilchromium is rendered essentially unavailable to be leached or otherwisetransported through the soil. It will be appreciated that, for manymetals, the Brassicaceae may act as both phytoaccumulators andphytoreducers. Although some growing and harvesting practices suitablefor phytoaccumulation are also suitable for phytoreduction, there areseveral soil/plant management practices that are different. For example,it is not recommended to added chelating agents such asethylenediaminetetraacetic acid (EDTA) or its analogs to soil in orderto enhance phytoreduction of Cr(VI). This is because addition ofchelators would likely bind Cr(III), thus increasing its solubility andpossibly increasing its oxidation to Cr(VI), defeating the purpose ofthe present reduction methods.

The chromium that can be reduced according to the methods of the presentinvention includes stable chromium and radioactive anionic chromium. Theterm "chromium" also includes mixtures of Cr(VI) and common organicpollutants such as, for example, lead or Cr(VI) in combination withnitrophenol, benzene, alkyl benzyl sulfonates (detergents),polychlorinated biphenyls (PCB's) and/or halogenated hydrocarbons (e.e.,trichloroethylene).

The preferred plants used in the present phytoreduction methods aremembers of the plant family Brassicaceae, described in detailpreviously. The most preferred members of this family belong to thetribe Brassiceae. Members of this tribe include mustards of the genusBrassica and related species.

A key aspect of the present invention is that the preferredphytoreduction methods rely upon use of crop and/or crop-related membersof the above-identified family and tribe. The term "crop member" has thesame definition as discussed above with regard to phytoaccumulation.Examples of "vegetative" crop members of the family Brassicaceaeinclude, but are not limited to, digenomic tetraploids such as Brassicajuncea (L.) Czern. (mustard), B. carinata Braun (ethopian mustard), andmonogenomic diploids such as B. oleracea (L.) (cole crops), B. nigra(L.) Koch (black mustard) and B. campestris (L.) (turnip rape). Examplesof "oil-seed" crop members of the family Brassicaceae include, but arenot limited to, B. napus (L.) (rapeseed), B. campestris (L.), B. juncea(L.) Czern. and B. tounifortii. "Crop-related" members include membersof species belonging, but not limited to, genera of Sinapsis, Thlaspi,Alyssum, Eruca and Raphanus. "Crop-related" members have the identicaldefinition as discussed above with regard to phytoaccumulation."Crop-related" members not presently identified, or suspected ofremoving chromium, can be identified using the screening methodsdescribed herein. Unless indicated otherwise, "crop and/or crop-related"members will be referred to collectively as "members".

The Cr(VI)-containing environment into which these plants are introducedis not intended to limit the scope of the invention. That is, as long asthe environment can sustain growth of members of the familyBrassicaceae, the Cr(VI)-containing environment includes a wide range ofsoil environments of varying degrees of water saturation, organic mattercontent, mineral content, and the like, including complete hydroponicgrowth using fluid-filled reservoirs to supply all the plants dissolvedand particulate nutrient needs. It will be appreciated by those ofordinary skill in the art that the term "soil" can, therefore, include awide variety of chemical and physical types.

The Cr(V)-reducing members suitable for the present methods will reduceCr(VI) in the roots of the plant. The plants may then translocate thereduced Cr(III) from the roots into the shoots (i.e., the above groundportions of the plant). The rates of Cr(VI) reduction can vary dependingon a variety of factors, including the ratio of reduced and oxidizedchromium in the soil, the total anionic metal concentration, soil type,pH, moisture content, organic matter content, soil temperature, plantingdensity, and fertilizer use. Shoots are routinely harvested for certainBrassica species, for example B. campestris, B. juncea and B. oleracea,but roots are not routinely harvested, especially for most oil-seedBrassicaceae.

It will be appreciated that those having ordinary skill in the art mayapply the procedures detailed herein to other plants besides members ofthe Brassicaceae and to other metals such as vanadium, selenium andarsenic that show more than one oxidation state in soil.

To measure metal (e.g. chromium) reduction potential of any plant in anychromium-containing soil, seeds of the particular plant(s) to be testedare grown in a greenhouse, the appropriate metal (e.g., chromium) isadministered to the plant and soil, and the roots and shoots harvestedfor routine determination of biomass and metal content. Chemicalanalysis of metal content in soils and plants is well-characterized.See, for example, Blincoe et al., Comm. Soil. Plant Anal., 18: 687(1987); Baker, D. E. and Suhr, S. H., "Atomic Absorption Spectrometry",pp. 13-27 in Methods of Soil Analysis, part 2, Am. Soc. Agron., Madison,Wis., (1982). Metal in plant tissues is preferably assayed with plasmaspectrometry, following ashing and acid extraction. Metal remaining inthe solution is measured by, for example, atomic absorption or plasmaspectrometry. See, Soltanpour et al, "Optical emission spectrometry",pp. 29-65 in Methods of Soil Analysis, part 2, Am. Soc. Agron., Madison,Wis., (1982). The oxidation state of the particular metal in the plantmay be measured using conventional techniques such as X-ray absorbancespectroscopy, XAS (See Example 8), which is the least invasive techniqueand provides the most relevant data for in vivo conditions.Ion-chromatography can also be used to quantify Cr(III) and Cr(VI)although the sample needed is much larger than for XAS. Thus, otherplants besides the Brassicaceae may be tested for their ability toconvert Cr(VI) to Cr(III).

The invention described herein will be illustrated by the followingexamples.

EXAMPLE 1

Phytoaccumulation

Growing and Harvesting Practices

The present Brassica growing methods include practices suitable forincreasing the amount of metal accumulated by the plant; practices verydifferent from those used to grow Brassicas for food, fodder or oils.

A. Soil Preparation Methods

Conventional agronomic practices for Brassicas involve soil tilling downto a maximum depth of about 5-8 cm (2-3 inches) in India and 10-13 cm(4-5 inches) in Canada. See, for example, "Canola and Rapeseed:Production, Chemistry, Nutrition, and Processing Technology", ed. M.Shahidi, Van Nostrand Reinhold, New York, 1990, and Canola GrowersManual, Canada Council of Canada, 1984, p. 703, both of which areincorporated herein by reference. Experiments have shown that tillagebelow 15-18 cm (6-7 inches) is of little value. In contrast, presentmethods of the invention involve tilling soil in a manner that purposelyexposes the Brassicaceae root zone to maximum amounts ofmetal-contaminated soil. This is accomplished by tilling the soil todepths greater than 2 cm and as far down as 50 cm. Conventionalimplements may be employed for this purpose, provided that they aresuitable for tiling down to the depths required by the present methods.These implements include moldboard plows, chisel plows, tandem andoffset disc plows, and various harrowers known to those having ordinaryskill in the art. The exact implement used will depend on factors suchas soil moisture, soil texture, weed cover and the like.

B. Soil Treatments to Increase Metal Availability to Plants

Metal uptake by plants is considered highly undesirable because metalcontaminated plants are toxic to humans or animals. Nevertheless, theability of crop and crop-related Brassicaceae to accumulate meal inshoots and roots is directly related to metal availability in soils. Thepresent metal-remediating plants are therefore used in combination withsoil treatments or amendments which make metals in soils more availableto the roots of the plants.

In one embodiment, one or more metal chelating agents are added to thesoil in amounts sufficient to increase metal mobility but not sufficientto affect plant growth and development (i.e., an "effective" amount).Determination of effective amounts of chelating agent may be made bymeasuring the effects of soil amendments of chelator on soil metalmobility. Soluble metals are extracted from soil by equilibrating about5 g of soil with about 25 ml of 0.01 M calcium nitrate (to maintainionic strength) for about 2 hours on a mechanical shaker. After theequilibration period, the suspension is centrifuged (between3000-5000×g) for about 15 minutes to separate the solution from thesoil. The supernatant solution is then analyzed for the desiredwater-soluble metal concentration. See, for example, Mench et al., J.Environ. Qual. 23: 58-63 (1994). Measured metal concentration iscorrelated with the various soil amendments to optimize conditions inorder to maximize metal solubility in the soil and maximize plantavailability.

The amount of chelating agent, and the time of administration of thechelating agent will vary, being primarily a function of the amount ofmetal in the soil and soil type. Many chelating agents will form solubleor partially soluble complexes with metal ions which can make the metalmore available to the plants and allow Brassicaceae to accumulate aparticular metal. Exemplary metal chelating agents of this are given inDamson et al., (eds), "Stability Constants of Metal Complexes", pp.399-415 in Data for Biochemical Research, Claredon Press, Oxford, UK,1986, incorporated herein by reference and include ammonium purpurate(murexide), 2,3-butane-dione dioxime (dimethylglyoxime), 3,6disulfo-1,8-dihydroxynaphthalene (chronotropic acid), and thiourea,alpha-benzoin oxime (cupron), trans-1,2-diaminocyclohexanetetraaceticacid (CDTA), diethylenetriaminopentaacetic acid (DTPA),2,3dimercapto-1-propanol, diphenylthiocarbazone, nitrilotriacetic acid(NTA), substituted 1,10-phenanthrolines (e.g., 5-nitro-1,10phenanthroline), sodium diethyldithiocarbamate (cupral),2-thenoyl-2-furoylmethane, thenoyl-trifluoroacetone,triethylenetetramine, and ethylenediaminetetraacetic acid (EDTA) andcitric acid. Other chelating agents may form insoluble complexes withmetals and serve to: (i) concentrate metals so they may be physically orchemically accumulated (i.e., sorbed) onto roots of the plants; and/or(ii) prevent leaching or other removal of metals from the vicinity ofthe root zone.

Chelating agents are preferably applied to the soil by conventionalirrigation pipes or other ground level irrigation systems. Chelatingagents may also be applied through other commercially availablefertilizer and chemical application equipment, including large volumesprayers. Chelating agents may be applied through broadcast methods forlarge areas or banding methods for the root zone. Chelating agents arepreferably applied at concentrations from 0.1-10 mmol/kg soil at volumesranging from about 5 to 200 tons per acre.

Further treatments are designed to increase metal mobility in the soilby decreasing soil pH. Conventional methods of growing Brassicaceaegenerally require soil in the pH range 5.8-6.2 for optimum productionand the available literature suggests that soils with lower pH bespecifically amended with base (e.g., lime) prior to seeding to increasethe pH. See, "Agronomy of Canola in the United States", pp. 25-35 inCanola and Rapeseed, Production, Chemistry, Nutrition, and ProcessingTechnology, (ed. F. Shahidi), Van Nostrand Reinhold, New York, 1990,incorporated herein by reference.

In preferred methods of the present invention, pH of themetal-contaminated soil is dropped to about pH 4.5-5.5 by acidifyingsoil with effective amounts of organic or inorganic acids such as nitricacid, hydrochloric acid, sulfuric acid, acetic acid and citric acids.Acids are preferably applied to the soil by conventional irrigationpipes or other ground level irrigation systems. However, acids may alsobe applied through other commercially available fertilizer and chemicalapplication equipment, including large volume sprayers. Acids may beapplied preferably at concentrations from 0.1 mM to 1.0 M at volumesranging from about 5 to 200 tons per acre or at levels sufficient todrop soil pH in the plant rhizosphere (down to about 40 cm) to between4.5 and 5.5 pH units.

Further, compounds may be added to the soil that depress soil pH becauseof biological activity of roots and microorganisms. Examples of thesecompounds include urea or ammonium sulfate. This so-called "biologicalacidification" occurs because the positively charged ammonium ions thatare incorporated into the roots and/or microorganisms are replaced withpositively charged protons exuded or otherwise released from therhizosphere into the soil, thus lowering the soil pH. Theammonium-containing compounds are applied at 0.5 to about 2.0 tons peracre.

Yields of Brassicaceae will be reduced at pH levels below about 5.5 butthe present methods require a balance between reduction in growthpotential due to increased soil acidity and increase in availability ofmetals in metal-contaminated soils due to lowered pH. Most preferably,the methods described herein weigh this balance in favor of increasingmetal availability so that the Brassicaceae will accumulate the metal.

Metal uptake may be further enhanced by using electrical fields toincrease metal mobility. See Probstein and Hicks, Science 260: 498-503(1993), incorporated herein by reference. In these general methods, adirect current electric field is applied across electrode pairs placedin the ground. The electric field induces motion of liquids anddissolved ions.

Metal availability may be further enhanced by rapidly dropping soil pHby at least 2 pH units over a period of several days by adding strongchelators or acids prior to harvest but after the plants have reachedthe harvestable stage. This treatment is designed to decrease soil pH tobetween 3-4.5. Acids are preferably applied to the soil by conventionalirrigation pipes or other ground level irrigation systems or othercommercially available fertilizer and chemical application equipment,including large volume sprayers. Acids are preferably applied in amountssufficient to drop soil pH below 4.5 and concentrations may range from0.2 mM to 0.2 M at volumes ranging from about 5 to 200 tons per acre.Chelates are applied at levels that cause visible phytotoxicity andgrowth retardation in plants. Although such harsh treatment may slow orarrest plant growth or even kill the plants, higher levels of metalaccumulation in roots and shoots is expected before the plants stopgrowing and/or die. In addition, the harvesting of already killed plantsis easier since less weight needs to be handled.

C. Foliar Fertilization

Commonly used practices for Brassica cultivation involve use ofnitrogen/phosphorous/potassium fertilizers broadcast into the soil. See,for example, "Canola Growers Manual", Canola Council of Canada 1984, andYusuf and Bullock, J. Plant Nutrition, 12: 1279-1288, 1993, incorporatedherein by reference. Addition of phosphates to the soil leads toformation of insoluble complexes with heavy metals and iscounterproductive in the present context since phosphate additionreduces the availability of heavy metals to plants and decreasesefficiency of metal remediation.

To prevent addition of phosphate to soils while maintaining adequateplant nutrition, crop and crop-related Brassicas are foliar fertilizedwith, for example, ammonium phosphate. Foliar fertilization refers tothe spraying of nutrient solutions to the foliage in which the nutrientsare absorbed into the plant through the leaves. For macronutrients suchas nitrogen and phosphorous, the quantities that can be absorbed are toosmall to be of much use in large-scale commercial agronomic practices.See "Canola Growers Manual", id. Nevertheless, for purposes of soilmetal remediation, foliar addition of phosphate, and soil addition ofnitrogen, will reduce addition of phosphates directly to soil whilemaintaining adequate plant nutrition. Brassicaceae grown for metalaccumulation are sprayed with a foliar fertilizer having a highphosphorous content (e.e., non-ammonium phosphate at a rate of 10-18kg/hectare). Foliar fertilizers may be applied via conventionalpesticide sprayers.

D. Harvesting before seed maturation

Conventionally, oil-seed brassicas are grown to maturity and the seedsare harvested. See Shahidi, supra. In contrast, oilseed plants used inthe present methods are harvested at the vegetative state, before theseeds mature. The exact harvest time is determined by selecting a plantage which provides maximum metal removal capacity from unit surfacearea.

E. Harvesting only Shoots and Roots

Conventionally, oil-seed brassicas are grown for their seeds andvegetative brassicas are grown for their leaves or stems. Thus, forhighest yields of Brassicaceae, particularly the oil-seeds, the plantsneed to be swathed, i.e., removed above the ground level, exactly likecereal crops. Conventional swathers and combines are used in theseprocedures. Nevertheless, much of the metal taken up by oil-seed andvegetative brassicas is concentrated in the roots (See also Examples 5and 6). Therefore, methods of the invention involve harvesting roots,either manually or by adapting conventional agricultural machinery usedfor beet, carrot or potato harvesting. Metal-containing soil at depthsgreater than about 20 cm may be excavated and filed into hills alongsidethe furrows. Brassicaceae are grown directly on the excavated hills.Although this is conventional technology for root crops such as carrots,beets, and potatoes, Brassicaceae are not normally grown in this manner.Nevertheless, the roots of Brassicaceae, especially the oil-seedbrassicas, may be easily harvested from the hills by undercutting theroots with a blade or plough.

EXAMPLE 2

Phytoreduction

Growing and Harvesting Practices

A. Vegetative "Plowing-Under"

Soils contaminated with highly mobile Cr(VI) may be remediated throughintensive cultivation of several annual crops of fast growing plantssuch as B. juncea which will effectively convert Cr(VI) in the soil tothe much more environmentally benign form, Cr(III) in the plant. This isaccomplished without removing plants from the field. Brassica species ofthe invention, still growing and flowering (i.e., 2-3 months old) areplowed under and the field immediately replanted with more plants.

Plowing plants back into the soil after growth is not a conventionalagronomic procedure but in this context, the plowed plants will helpincrease the soil organic content which will assist in reducingoxidation of Cr(III) back to Cr(VI). Increasing the organic content ofthe soil in this way may also be supplemented with additions of manure,leaf litter, and the like (Bartlett and Kimble, J. Environ. Qual. 5:382-386, 1978). Addition of ferrous ion Fe(II)! will also help preventthis oxidation.

To further increase the availability of Cr(VI) for plant uptake,phosphate can be added. Amending the soil with phosphate will also tendto form precipitates of Cr(III) as chromium phosphate, thus furtherreducing the possible oxidation of Cr(III) back to Cr(VI).

EXAMPLE 3

Screening Assays for Phytoaccumulators

The seeds of crop and/or crop-related species of selected members of theBrassicaceae are sown in a potting mix (Terralite™ Metro-Mix™; mfg. byGrace Fiera Horticultural Products Co., Milpetas, Calif.) and grown in agreenhouse equipped with supplementary lighting (16 h photoperiods; 24°C.). Seedlings are fertilized every two days with a full strengthHoagland's solution. After 10 days the seedlings are transplanted (twoper 3.5 inch plastic pot) into an acid pre-washed 1:1 (v/v) mixture ofcoarse sand and coarse Perlite.

During a 7-day long period of establishment, seedlings are well-wateredand fertilized with KNO₃ solution. Thereafter, aqueous solutions of leadin the form of Pb(NO₃)₂ or chromium in a form of K₂ Cr₂ O₇ areadministered to the surface of the growing medium to obtain 625 ug Pb⁺²or 3.5 ug Cr⁺⁶ per gm of dry soil. After the metal application, plantsare irrigated with water only. Control plants are watered from the topwith KNO₃ solution on the day of metal treatment to deliver the sameamount of NO₃ ⁻¹ or K⁺¹ as the salts of metals. For all treatments theexcess soil moisture is trapped in 4 inch plastic saucers placed beloweach pot. Roots and shoots of treated and control plants are harvested12 to 20 days after the metal treatment. Metal content, dry matteraccumulation, and metal-related toxicity in treated plants is determinedand compared to the untreated control. Metal content of roots and shootsis measured by direct current plasma spectrometry.

In an interspecies screen summarized below, lead uptake by members ofthe Brassiceae tribe (*) was compared with non-Brassica plants and witheach other (Table 1).

                  TABLE 1    ______________________________________    Lead-accumulating capacities of different members of Brassiceae    tribe (*). The experiment was repeated with similiar results.    Standard error did not exceed 30% of the mean. Lead content    (μg Pb.sup.+2 /g dry weight).                       Days after treatment    Plant        Tissue      12      20    ______________________________________    B. juncea *  Shoot       9,346   18,812                 Root        70,090  91,666    B. carinata *                 Shoot       1,856   8,757                 Root        76,815  115,461    B. nigra *   Shoot       1,439   2,489                 Root        29,671  110,011    B. campestris *                 Shoot       1,242   6,941                 Root        22,651  100,354    B. oleracea *                 Shoot       2,333   1,416                 Root        51,420  51,399    B. napus *   Shoot       5,720   3,877                 Root        68,117  60,768    Sinapis arvensis *                 Shoot       --      498                 Root        --      42,660    Raphanus sativus *                 Shoot       --      886                 Root        --      44,157    Nicotiana tabacum                 Shoot       --      786                 Root        --      24,872    Sorghum bicolor                 Shoot       --      280                 Root        --      14,693    Zea mays     Shoot       --      236                 Root        --      8,165    ______________________________________

Of all the species studied, Brassica juncea was the best accumulator oflead in shoots, accumulating lead 30-fold over soil values. B. carinataaccumulated the highest levels of lead in roots, accumulating lead about185-fold over soil values. In general, all species of brassicasaccumulated exceptionally high levels of lead in shoots and roots. Othermembers of the Brassiceae tribe were also good accumulators of lead whencompared to species belonging to different taxonomic groups. Over ninetypercent of lead present in the shoots of B. juncea grown onlead-containing medium for 12 days was present in the stems andreproductive tissue. Leaves contained smaller amounts of lead on a dryweight basis.

In addition to having the highest accumulating ability in the shootportions, B. juncea showed low lead toxicity. It is also known to be ahigh biomass producer (average yield of 18 tons/hectare: See, Bhargava,S. C., "Physiology", pp. 161-197 in Oilseed brassicas in IndianAgriculture, (eds. Chopra, V. L. and Prakash, S.), Vikas PublishingHouse Ltd, New Delhi, (1991)).

EXAMPLE 4

Selection of Phytoaccumulating Brassica cultivars

Identification of B. juncea as the best shoot accumulator (see Example2) allowed an exhaustive screening of 120 B. juncea cultivars hoping toutilize existing genetic variability and find the best metal-extractingcultivars. B. juncea cultivars originating from 4 different continentswere obtained from Dr. Peter K. Bretting, USDA/ARS, Iowa StateUniversity, Ames, Iowa 50011. The screening methods described in Example2 were used throughout. Seedlings were exposed to 625 micrograms leadper gram dry weight soil for 14 days.

FIG. 1 demonstrates the ability of the ten best lead-accumulatingcultivars of B. juncea identified in our screen to concentrate lead inshoots (A) and roots (B). Cultivar 426308, the best shoot accumulatoridentified so far, accumulated almost 55-fold lead in the dried shoots,a lead concentration of 3.5%. Moreover, roots of 426308 were able toconcentrate lead 173-fold over the lead levels in the growing medium.This is equivalent to about 10% by weight of lead in the dried roots.The highest root accumulation was, however, observed in lines 211000,478326 and 478336. These cultivars concentrated lead in their rootsabout 320-fold, 480-fold, and 350-fold, respectively, equivalent toabout 20%, 30% and 25% by weight of lead in the dried roots. Alllead-accumulating cultivars are vigorous plants with high biomassproduction--another important trait for a plant to be used for metalextraction.

In chemical engineering terms, Brassica juncea roots can performchemical precipitation and are an extremely effective ion exchange resinand stabilize lead in the soil, as demonstrated below. Twelve day oldseedlings of Brassica juncea cultivar 173874 are transplanted in groupsof 15 plants each into 3.5" pots with a sand/Perlite mixture (150 g/potby dry wt.), placed in 4.5" plastic saucers, and allowed to grow furtherfor 20 days. At the same time, pots containing the same amounts ofsand-Perlite mixtures but without plants are maintained as controls.Each pot (with and without plants) is watered on alternate days with 100mL of tap water. In addition, 30 milliliters of full strength Hoaglandnutrient solution are added weekly to each pot. At the end of the 20thday after transplanting, pots with and without plants are flushed with10 volumes of tap water. Thereafter, the lead solution is administeredinto each pot to obtain the final lead concentration of 625 ppm in drysoil. Control pots and pots with plants are watered with tap water everyother day.

On the sixth day after lead treatment, 1 ml of solution leached fromeach pot into the plastic saucer is collected to determine the availablelead by atomic absorption spectroscopy. As shown in FIG. 2, in the potswithout plants, between about 700-800 ppm lead was measured in theleachate. In the pots with plants, less than 20 ppm lead was measured.The experiment indicates that B. juncea roots can effectively make leadmuch more difficult to leach from the soil.

EXAMPLE 5

Phytoaccumulation of Chromium (VI)

Analysis of hexavalent chromium

S-diphenylcarbazide (0.3 g) was dissolved in 100 ml of 95% ethanol.Concentrated phosphoric acid (120 ml) was diluted to 400 ml withdistilled water and added to the s-diphenylcarbazide/ethanol solution.Hexavalent chromium was determined by the addition of 1 ml ofs-diphenylcarbazide/phosphate buffer solution to 10 ml of sample,followed by vortexing. Samples were incubated for 20 min at roomtemperature and the absorbance at 540 nm measured.

Chromium is much more toxic to plants than lead. Eighteen microgramsCr⁺⁶ /g DW soil, supplied for 20 days, were lethal for all testedBrassica species. Lethality is determined by observations of plantwilting and death by the end of the treatment. Tissue chromium ismeasured by plasma spectrometry following ashing and acid extraction.See Soltanpour et al, supra. At low concentrations of Cr⁺⁶ (about 3 toabout 9 micrograms Cr⁺⁶ /g soil), crop-related brassicas are extremelygood accumulators of this metal. In particular, both B. juncea and B.oleracea are excellent accumulators of Cr⁺⁶ (Table 2). For example, B.juncea cultivar 21100 concentrated chromium in its roots 650-fold and inits shoots 90-fold. Therefore, the accumulating capacity for chromium inboth shoots and roots of Brassicaceae species is even higher than forlead. B. juncea is likely better suited for chromium remediation than B.oleracea because of its higher biomass production and ease ofcultivation.

                  TABLE 2    ______________________________________    Phytoaccumulation capacities of B. oleracea and B. juncea    exposed to 3.5 and 8.5 μg Cr.sup.+6 /g dry soil for 20 d.    Plant chromium content is expressed as microgram total Cr.sup.+6 /g    dry weight tissue.                     μg Cr.sup.+6 /g    Plant        Tissue    3.5       8.5    ______________________________________    B. oleracea  Shoot     --         3 ± 53                 Root      --        2578 ± 204    B. juncea cultivars:    Rcb J*       Shoot     --        398 ± 43                 Root      --        1705 ± 136    182921       Shoot     226 ± 64                                     --                 Root      1834 ± 35    211000       Shoot     334 ± 112                                     --                 Root      2265 ± 239    173874       Shoot     182 ± 81                                     --                 Root      1945 ± 7    ______________________________________     *Rcb Jobtained from Crucifer Genetics Cooperative, Madison, WI.

Roots of 3-week old B. juncea seedlings, grown in 500 ml of solutionculture (25% Hoagland's solution), were exposed to 30 micromolar Cr(VI)as the hexavalent dichromate anion. FIG. 4 illustrates that Cr(VI) wasrapidly removed from solution, accumulating mainly in the roots.

EXAMPLE 6

Phytoaccumulation of both Anionic and Cationic Metals

This Example illustrates phytoaccumulation of both anionic and cationicspecies of metal by the same plant.

Seedlings of Brassica juncea and Amaranthus paniculata are grown in 3.5"pots with a sand/Perlite mixture (150 g/pot by dry wt.), placed in 4.5"plastic saucers, and allowed to grow for 21 days at which pointsolutions of different metals were added to the soil. Between 2-500micrograms of metal/gram soil were applied. Each pot is watered onalternate days with 100 mL of tap water. In addition, 30 milliliters offull strength Hoagland nutrient solution are added weekly to each pot.Metal concentration in roots and in soil is measured 14 days afteraddition of metals. A metal accumulation potential is calculated bydividing metal concentration in root tissue on a dry weight basis tometal concentration in soil, on a dry weight basis. FIG. 3 illustratesthat both plants will accumulate cationic cadmium, anionic chromium (VI)and cationic zinc, but that B. juncea will accumulate cationic nickel aswell.

EXAMPLE 7

Screening Assay for Phytoreducers

The seeds of crop and/or crop-related species of selected members of theBrassicaceae are sown in a potting mix (Terralite™ Metro-Mix™; mfg. byGrace Fiera Horticultural Products Co., Milpetas, Calif.) and grown in agreenhouse equipped with supplementary lighting (16 h photoperiods; 24°C.). Seedlings are fertilized every two days with a full strengthHoagland's solution. After 10 days the seedlings are transplanted (twoper 3.5 inch plastic pot) into an acid pre-washed 1:1 (v/v) mixture ofcoarse sand and coarse Perlite.

During a 7day long period of establishment, seedlings are well-wateredand fertilized with KNO₃ solution. Thereafter, aqueous solutions ofmetal capable of being, or suspected of being phytoreduced (e.g.,anionic hexavalent chromium such as in the form of K₂ Cr₂ O₇) areadministered to the surface of the growing medium to obtain severalmicrograms of metal ion per gm of dry soil. After the chromiumapplication, plants are irrigated with water only. Control plants arewatered from the top with KNO₃ solution on the day of metal treatment todeliver the same amount of NO₃ ⁻¹ or K⁺¹ as the salts of the metal. Forall treatments the excess soil moisture is trapped in 4 inch plasticsaucers placed below each pot. Roots and shoots of treated and controlplants are harvested 12 to 20 days after the metal treatment. Metalcontent, dry matter accumulation, metal-related toxicity, and oxidationstate of metal in treated plants is determined and compared to theuntreated control. Metal content of roots and shoots is measured bydirect current plasma spectrometry. The screening assay may also beperformed hydroponically, as illustrated in Example 7 for hexavalentchromium.

EXAMPLE 8

Phytoreduction of Cr(VI) to Cr(III) in plants

A. Plant material

B. juncea was grown hydroponically, with continuous aeration, in agrowth chamber with a day/night cycle of 16 hr/8 hr at about 25° C. Thehydroponic medium (a diluted and modified solution after Hoagland andArnon, 1938) contained the following nutrients: 1 mol m⁻³ ammoniumphosphate monobasic, 0.05 mmol m⁻³ boric acid, 2.8 mol m⁻³ calciumnitrate, 0.3 mmol m⁻³ copper sulfate pentahydrate, 0.019 mol m⁻³ ferrictartarate, 2 mol m⁻³ magnesium sulfate anhydrous, 9 mmol m⁻³ manganesechloride tetrahydrate, 0.1 mmol m⁻³ molybdenum trioxide, 6 mol m⁻³potassium nitrate, and 0.8 mmol m⁻³ zinc sulfate heptahydrate. Threeweek old seedlings were exposed to Cr(VI), as the dichromate anion (Cr₂O₇ --) in nutrient solution. At the end of the experimental period rootsused for X-ray absorbance spectroscopy (XAS) were rapidly frozen inliquid nitrogen and stored at -80° C. Prior to performing XAS, frozenroots were ground in liquid nitrogen and the frozen powder loaded intoXAS sample cells.

B. X-ray absorbance spectroscopy (XAS)

Chromium K-edge X-ray absorption spectra were collected on beamline 7-3at the Stanford University Synchrotron Radiation Laboratory, with thestorage ring SPEAR operating at 3 GeV with ring currents of 500-100 mA.Si(220) monochromator crystals were used, with an upstream verticalaperture of 1 mm, and no focusing optics. The monochromator crystalswere 50% detuned at the absorption edge, in order to reject harmonics.X-ray absorption was monitored by recording the fluorescence excitationspectra using a Canberra 13-element Ge array detector, or by X-raytransmittance using nitrogen-filled ionization chambers. The former wasused for plant samples, and the latter for the standard compounds (SeeFIG. 5). Samples were held at a constant temperature in the range 4-8degrees K using an Oxford Instruments CF1204 liquid helium flowcryostat.

Results are presented in FIG. 5 which illustrates the XAS of B. juncearoots exposed to solutions of Cr(VI). Samples of roots and leaves wereanalyzed as described above and the XAS compared with standardsincluding chromium as: (i) Cr(VI) in the form of K₂ CrO₄ ; (ii) Cr(III)in the form of Cr(NO₃)₃ and Cr₂ O₃ ; and (iii) elemental chromium. Theabsorbance peak at about 5990 KeV is a marker for hexavalent Cr(VI) andthis peak is lacking in the oxidation states of chromium in the leavesand roots. The absorption peak at about 6005 KeV is indicative ofCr(III) and is shown as the dotted vertical line in the Figure. Underthe conditions of the experiments described herein, all the Cr in theplants is Cr(III) and there is 100% conversion of Cr(VI) to Cr(III).

EXAMPLE 9

EMS Mutagenesis

This example illustrates a protocol for use in mutagenizing plantmembers of the family Brassicaceae.

1. Dry seeds are placed in about 100 ml of a 0.3% (v/v) solution of EMS(obtained from Sigma chemicals, St. Louis, Mo.). There may be somevariation from batch to batch of EMS so it may be necessary to adjustthis concentration somewhat. Between 20,000 to 250,000 seeds aremutagenized at a time. Ethyl methane sulfonate (EMS) is a volatilemutagen. It should be handled only in a fume hood and all solutions andmaterials which it contacts should be properly disposed of.

2. Seeds are mixed occasionally or stirred on a stir plate and left atroom temperature for 16-20 hours. The rate of mutagenesis may betemperature-dependent so using a magnetic stir plate may alter theresults by warming the solution.

3. Seeds are washed with distilled water 10 to 15 times over the courseof 2 to 3 hours by decanting the solution, adding fresh water, mixing,allowing the seeds to settle, and decanting again. After about 8 washesthe seeds are transferred to a new container and the original isdisposed of.

4. After washing, the seeds are immediately sown at about 1 seed persquare cm (3000 seeds in 50 ml of 0.1% agar per 35×28×9 cm flat).

5. After several weeks it is useful to estimate the number of seedswhich have germinated in order to know the size of the M1 generation.About 75% of the mutagenized seeds usually germinate. Ideally, the M1estimate is the number of plants which produce M2 seed, but this is muchmore difficult to measure.

6. Plants are grown until they begin to die naturally and are thenallowed to dry completely before harvesting. Complete drying improvesthe yield and simplifies harvesting.

EXAMPLE 10

Vector construction and transformation of B. juncea with MT genes

A. Vector Construction

Monkey MT cDNAs (MT1 & MT2) are obtained from Dr. Dean H. Hamer,National Institutes of Health, Bethesda, Md. A 341 bp Hind III/Bam HIfragment containing the entire MT1 coding sequence including theinitiator methionine codon is cloned into the Hind III/Bgl II site ofpJB90 to give plasmid pNK1. pJB90, a derivative of pGSFR780A (a giftfrom Dr. Deepak Pental, Tata Energy Research Institute, New Delhi,India) is an Agrobacterium based binary, plant transformation/expressionvector. This plasmid contains a plant selectable hpt (hygromycinphosphotransferase) gene and a multiple cloning site for the insertionof foreign DNA, between the T-DNA border repeats. The plasmid alsocontains a gene for spectinomycin resistance, functional in bacterialcells. pNK1 propagated in E. coli Dh5 was used to transformAgrobacterium tumefaciens strain pGV2260 (Deblaere et al., Nucl-AcidsRes., 13:4777 1985) by the freeze-thaw method (Ebert et al., PNAS,U.S.A., 84:5745 1987).

B. Transformation of B. juncea

The 10 best hyperaccumulating B. juncea lines--173874, 182921, 211000,250133, 426314, 426308, 531268, 537004, 537018--were selected fortransformation.

Agrobacterium tumefaciens strain pGV2260 carrying pNK1 is grownovernight (220 rpm, 28° C. in dark) in 5 mL of liquid YEB (0.5% beefextract; 0.1% yeast extract; 0.5% peptone; 0.5% sucrose; 0.005%MgSO₄.7H₂ O in distilled water) containing 100 mg/L each ofspectinomycin and rifampicin. One mL of this suspension is used toinoculate 50 mL of the YEB with the same concentrations of antibioticsand allowed to grow overnight. On the third day, the bacteria areharvested by centrifugation (5500 rpm) and resuspended in filtersterilized liquid MS (see Murashige, T., and Skoog, F., Physiol. Plant,15: 473-497 (1962)) modified medium (MS salts & vitamins with 10 g/Leach of sucrose, glucose and mannitol) supplemented with 200 micromolaracetosyringone and 100 mg/L each of spectinomycin and rifampicin at pH5.6. The optical density of the bacterial suspension is adjusted toabout A₆₀₀ =1.0 and the bacteria grown for 6 hours, harvested as beforeare resuspended in the same medium. Freshly cut hypocotyl explants areincubated in the bacterial suspension for 1 h and co-cultivated on MSmodified medium supplemented with 2 mg/L BAP (6-benzylaminopurine) and0.1 mg/L NAA (naphthaleneacetic acid). After 2 days the explants aretransferred to MS medium supplemented with 2 mg/L BAP, 0.1 mg/L 2,4-D(2-4 dichlorophenoxyacetic acid), 200 mg/L Cefotaxime and 30 micromolarAg(NO₃)₂ and 10 mg/L Hygromycin B. After 10 days incubation on thismedium, the explants are shifted to MS supplemented with 2 mg/LBAP, 0.1mg/L NAA, 200 mg/L Cefotaxime, 10 mg/L Hygromycin B and 10% coconutmilk. Shoots developed in 15-20 days are grown further and rooted in thepresence of 20 mg/L hygromycin. We have obtained transformants with theline 173874 at a frequency of about 2%.

C. Characterization of MT gene expression in transgenic plant lines

About 15 independent transgenic plants are generated for each B. juncealine mentioned above. The putative transformants are analyzed for thepresence of MT1 DNA by Southern and Northern hybridization analysisusing MT1 cDNA as a probe. The putative transformants are analyzed forexpression of MT1 protein by immunoblot analysis with antisera againstmonkey MT.

Transgenic lines expressing high MT levels are selected and tested forlead and chromium accumulation and metal tolerance in greenhouse trialsdescribed above. The transgenic lines are evaluated in large scalegreenhouse trials which will utilize lead and chromium contaminated soilcollected from the polluted sites.

Conclusions

The plant members described in the present invention represent adramatic improvement in the ability to accumulate metals because oftheir much higher total biomass accumulation than wild, non-crop-relatedmembers of the Brassicaceae described in the literature. For example, B.juncea on an average yields 18 tons/hectare of harvestable biomass(Bhargava, S. C., supra). This is an order of magnitude higher than canbe expected from the wild, non crop-related species of the Brassicaceaegrown under the most favorable conditions.

Based on the available information, the following calculation of therate of lead removal from contaminated soils can be made. Assuming totalabove-ground biomass production of 10 tons/hectare and 3.5% (dry weight)lead accumulation in plant shoots, one planting of the bestlead-accumulating lines of Brassica juncea (cultivar 426308) can removeas much as 350 kg lead/hectare. In most of the areas of the UnitedStates, 3 sequential crops of this plant can be grown each year.Therefore, the best metal-accumulating lines of crop brassicas selectedaccording to the methods of the present invention can extract one ton oflead per hectare per year. These estimates of the metal-removingcapabilities of crop-related plants of the present invention assume thatthe soils can be extracted to a depth of up to one meter, whichapproximates the depth to which the roots of crop-related members of thefamily Brassicaceae can reach under favorable conditions.

The most commonly used method for cleaning toxic metal-contaminated soilis removal and isolation which costs an average of about $400 per ton ofsoil. If the contamination is 80 cm deep in sandy loam soil having adensity of about 2.0 grams/cm., it will cost about $2.56 million toclean up one acre using this soil removal method.

The cost of growing the crop-related members of the Family Brassicaceaein the present invention may be approximated from the cost of alfalfaproduction in New Jersey which is about $320.00 per acre for the averagefarmer. Approximately 4.2 tons of dry plant matter per acre can bereduced to 40 kilograms of ash per acre if the plants are incinerated.Removing and burying that much plant residue will cost from about $640to about $1,680 per acre, making the total cost of one crop between $960and $2,000. Therefore, growing even ten sequential crops of the plantsdescribed in the present invention will be several orders of magnitudecheaper than a soil removal method. Furthermore, this method is betterfor the environment since it reclaims the soil making it usable ratherthan permanently disposing of the soil.

We have calculated that in 150 g soil, containing 525 micrograms Cr(VI),two B. juncea plants can convert 20% of the total Cr(VI) to Cr(III) in20 days.

Under some circumstances, the metal can actually be reclaimed from thehighly enriched plant ash. Concentrating the metal from plants afterharvesting may be accomplished either by direct smelting of the bulkplant matter or may incorporate a number of volume reduction stepsbefore the smelting process. Methods of reducing the bulk volume of theplant matter include incineration, anaerobic and aerobic digestion, aciddigestion or composting. The most preferred method of concentration is amethod that involves one or more of the above mentioned volume reductionmethods followed by direct smelting. Smelting of metal (e.g.,lead)-containing material is a technique well known in the art andvariations on the method are given in, for instance, "Lead Smelting andRefining: Its Current Status and Future" by M. Kazue, pp. 23-38T, inLead-Zinc 1990, Proc. World Symp. Metall. Environ. Control, T. S. Mackey(ed.); Mineral., Metal. Mater. Soc., Warrendale, Pa. (1990), hereinincorporated by reference.

Thus, post-harvest processing of the terrestrial plant material includesone or more steps that will result in the environmentally acceptablereclamation or disposal of the metal in the plant tissue. In the eventthat a pre-processing step is needed to increase metal concentration andbulk density, as well as to reduce the total volume, concentration ofthe terrestrial plant biomass may be accomplished by processes includingaerobic digestion (e.g. a compost pile), anaerobic digestion (e.g.,enclosed tank) incineration (e.g. aching), grinding, chopping,palliating, or wet chemical digestion (acid treatment). This willcompletely eliminate the need for residue burial and provide a trulyenvironmentally friendly remediation technology.

Equivalents

It should be understood that various changes and modifications of thepreferred embodiments may be made within the scope of the invention.Thus it is intended that all matter contained in the above descriptionbe interpreted in an illustrative and not limited sense.

What is claimed is:
 1. A method of converting chromium (VI) to chromium(III) comprising:selecting a soil environment contaminated with chromium(VI); and planting a member of the family Brassicaceae in said soilenvironment, said Brassicaceae member being capable of convertingchromium (VI) to chromium (III); and maintaining said Brassicaceaemember in said soil environment for a time and under conditionssufficient for said Brassicaceae member to convert chromium (VI) tochromium (III) by phytoreduction; and manipulating said soil environmentto inhibit oxidation of chromium (III) to chromium(VI), said step ofmanipulating comprising maintaining said Brassicaceae member in saidsoil environment.
 2. The method of claim 1, in which the step ofmanipulating further comprises:plowing said Brassicaceae member backinto said soil environment.
 3. The method of claim 1, in which the stepof manipulating further comprises increasing the organic content of saidsoil environment.
 4. The method of claim 2, further comprising, afterthe step of plowing said Brassicaceae member back into said soilenvironment:replanting said soil environment with a new Brassicaceaemember capable of converting chromium (VI) to chromium (III); andmaintaining said new Brassicaceae member in said soil environment for atime and under conditions sufficient for said new Brassicaceae member toconvert chromium (VI) to chromium (III) by phytoreduction.
 5. The methodof claim 1, wherein the step of manipulating further comprises amendingthe soil with phosphate.
 6. The method of claim 1, in which said step ofplanting comprises planting with a crop member of the familyBrassicaceae.
 7. The method of claim 6, in which said crop member isselected from the group consisting of Brassica juncea, Brassica oleraceaand Brassica carinata.
 8. The method of claim 7, in which said cropmember is a Brassica juncea plant.
 9. The method of claim 1, in whichsaid step of planting comprises planting with a crop-related member ofthe family Brassicaceae.
 10. The method of claim 9, in which saidcrop-related member is selected from the group consisting of Raphanussativus, Sinapis alba, Sinapis arvensis, Sinapis flexuosa, Sinapispubescens and Amaranthus paniculata.
 11. The method of claim 1, in whichsaid soil environment is further contaminated with pollutants selectedfrom the group consisting of lead, nitrophenol, benzene, alkyl benzylsulfonates, polychlorinated biphenyls and halogenated hydrocarbons. 12.The method of claim 1, in which said soil environment comprises afluid-filled reservoir, so that said Brassicaceae member is planted andmaintained hydroponically.
 13. The method of claim 1, further includinga step of manipulating said soil environment, which step of manipulatingsaid soil environment is performed prior to said step of maintaining andcomprises applying an electric field to said soil environment toincrease the mobility of chromium (VI).
 14. The method of claim 1,further including a step of manipulating said soil environment, whichstep of manipulating said soil environment is performed prior to saidstep of maintaining and comprises reducing the pH of said soilenvironment.
 15. The method of claim 14, in which the pH of said soilenvironment is reduced to approximately 5.5 or less.
 16. The method ofclaim 14, in which the step of reducing the pH of said soil environmentcomprises adding at least one acid selected from the group consisting ofacetic acid, citric acid, nitric acid, hydrochloric acid, and sulfuricacid.
 17. The method of claim 14, in which the step of reducing the pHof said soil environment comprises:selecting a compound that, whenexposed to a rhizosphere of said Brassicaceae member, is metabolized bysaid rhizosphere so that protons are produced; and adding said compoundto said soil environment so that said rhizosphere metabolizes saidcompound and protons are produced and released into said soilenvironment.
 18. The method of claim 17, in which said compound isselected from the group consisting of urea and ammonium-containingcompounds.
 19. The method of claim 1, further including a step ofmanipulating said soil environment, which step of manipulating said soilenvironment is performed prior to said step of maintaining and comprisestilling said soil environment to a depth containing a maximumconcentration of chromium (VI) prior to planting said Brassicaceaemember, so that, when planted, a Brassicaceae member root zone isexposed to a maximum concentration of said chromium (VI).
 20. The methodof claim 1, further comprising a step of exposing said plant tofertilizer.
 21. The method of claim 20, in which said fertilizer isselected from the group consisting of nitrogen fertilizers, phosphatefertilizers, potassium fertilizers, manure, and leaf litter.
 22. Themethod of claim 20, in which the step of exposing said plant tofertilizer comprises exposing said plant to fertilizer by utilizingfoliar fertilization techniques.
 23. The method of claim 1, furthercomprising a step of manipulating said soil environment to increase therate of reduction of chromium (VI) to chromium (III).
 24. The method ofclaim 1, in which the step of manipulating further comprises adding tosaid soil environment chemicals selected from the group consisting ofmanure, leaf litter and ferrous ion.
 25. A method of converting at leastone metal from a first oxidation state to a second oxidation statecomprising:selecting a soil environment contaminated with at least onemetal in a first oxidation state, the at least one metal being selectedfrom the group consisting of chromium, selenium, arsenic, and vanadium;and planting a plant in said soil environment, said plant being capableof converting said at least one metal from said first oxidation state toa second oxidation state by phytoconversion; and maintaining said plantin said soil environment for a time and under conditions sufficient forsaid plant to convert said at least one metal from said first oxidationstate to said second oxidation state by phytoconversion; andmanipulating said soil environment to inhibit conversion of said atleast one metal in said second oxidation state to said at least onemetal in said first oxidation state, said step of manipulatingcomprising maintaining said plant in said soil environment.
 26. Themethod of claim 25, in which said at least one metal is chromium. 27.The method of claim 25, in which said soil environment comprises afluid-filled reservoir, so that said plant is planted and maintainedhydroponically.